1. Field of the Invention
The present invention relates to a multilayer PTC thermistor.
2. Related Background Art
PTC (Positive Temperature Coefficient) thermistors are known as thermistors with positive resistance temperature characteristics, or increased resistance with increased temperature. PTC thermistors are constructed by forming at least a pair of opposing electrodes on a material having a positive temperature coefficient. PTC thermistors are utilized in self-regulating heating units, overcurrent protection elements, temperature sensors and the like.
As PTC thermistors there are known polymer PTCs comprising a body formed using materials obtained by mixing resins and metal powders. With polymer PTCs, the temperature coefficient of the resin is greater than that of the metal powder, and therefore the mutual distance between the metal powder increases as the temperature rises, such that the resistance of the body undergoes rapid increase. Since the temperature coefficient of resistance is large in such a polymer PTC, it is possible to reduce the maximum current during operation when it is used in an overcurrent protection element. However, when a polymer PTC is repeatedly operated, the resistance increases and the power consumption during non-operation tends to gradually increase.
Ceramic PTCs are also known, in which a ceramic body is formed using a ceramic material with a positive temperature coefficient. A ceramic PTC utilizes the fact that resistance drastically increases as the Curie point is approached. For example, a barium titanate-based dielectric material has a ferroelectric conversion point (Curie point) near 200° C. Even with repeated operation, a ceramic PTC allows resistance increase during non-operation of the PTC to be sufficiently inhibited compared to a polymer PTC. The PTC power consumption can thus be reduced.
In recent years, multilayer PTC thermistors have been employed which have increased electrode areas by embedding of laminar internal electrodes, as a strategy to reduce room temperature resistivity. A multilayer PTC thermistor has a ceramic body and external electrodes on the end faces of the ceramic body. In the external electrode, a film-like electrode layer is formed on the surface section by a plating method or thin-film process, in order to inhibit erosion from soldering during mounting. From the viewpoint of production cost, plating methods are superior to thin-film processes. However, because the ceramic body is porous, when wet plating is carried out to form a terminal electrode after the ground layer electrode has been formed, the plating solution infiltrates into the porous ceramic body. When this occurs, power fed from the internal electrode causes attachment of plating onto the interior of the ceramic body, which is a porous body, and over the entire surface, resulting in shorts between terminals.
Techniques of forming a coating layer such as a glass layer on the ceramic body surface are known for preventing infiltration of the plating solution into the ceramic body interior. For example, Japanese Unexamined Patent Application Publication No. 2004-128488 (Patent document 1) proposes forming an alkaline glass layer on the ceramic body surface to prevent infiltration of the plating solution into the ceramic body.